1. Field of the Invention
The present invention relates to a torque sensor and an electric power steering system.
2. Description of Related Art
In general, a torque detecting apparatus includes a Hall IC arranged between protruding portions protruding radially outward from ring portions of two, upper and lower magnetic collecting rings, and detects a change in generated torque as a change in a magnetic flux passing through the Hall IC. For a reduction in the thickness of a sensor unit used for the torque detecting apparatus, a technique has been developed in which a Hall IC used for torque detection is mounted on a printed circuit board such that a device portion of the Hall IC is generally parallel to the printed circuit board (see Japanese Patent Application Publication No. 2011-191094 (JP 2011-191094 A)).
Since the Hall IC mounted on the printed circuit board is arranged between the protruding portions (magnetic collecting portions) of the pair of magnetic collecting rings, the distance between the magnetic collecting portions of the pair of magnetic collecting portions tends to increase. An increased distance between the magnetic collecting portions of the pair of magnetic collecting portions progressively reduces the density of a magnetic flux passing through the Hall IC, thus progressively reducing detection accuracy. The reason is that the magnetic flux density is in inverse proportion to the square of the distance. JP 2011-191094 A describes a technique in which the device portion of the Hall IC is arranged in association with a cutout portion of the printed circuit board to improve permeability of the magnetic flux when the magnetic flux passes through the device portion of the Hall IC. However, the printed circuit board itself is originally not so low in the permeability of the magnetic flux. Therefore, the effect that significantly increases the magnetic flux density is not expected to be obtained simply by forming the cutout portion corresponding to the device portion of the Hall IC.